A multi-centre study on within-person variability in the urinary excretion of sodium, potassium, calcium, magnesium and creatinine in 8 European centres.

We have studied the within-person variability in the excretion of sodium, potassium, calcium, magnesium and creatinine in 20-60-year old men and women from 8 European centres. Only the data from people who reported that their collections were complete were entered in the analysis. The within-person coefficients of variation for the electrolytes ranged from 28 to 38 and that for creatinine from 21 to 24. The corresponding number of days required to estimate the excretion of electrolytes and creatinine to within 20 per cent of the habitual excretion (95 per cent confidence interval) varied between 4 for creatinine in men to 14 for magnesium in both men and women. The results of this study demonstrate once more that creatinine is unsuitable as a marker of completeness of urine collection for the individual.